For the larger grains (\(\tau _k \gg 1\) ), the same kind of change needs to be made to the Spitzer equation [1]}, [2]} that governs the grain's electric potential \(\psi \) . For \(\psi <0\) , \(e^\psi \)  represents the repulsion of the flux of electrons by the negatively charged grains, while \(1-\psi \)  characterizes the attraction of the flux of ions. The flux equilibrium can thus be written as
\((n_\mathrm {e}v_\mathrm {e}s_\mathrm {e})e^\psi = (n_\mathrm {i}v_\mathrm {i}s_\mathrm {i}+ n_\mathrm {s}v_\mathrm {s}s_\mathrm {s})(1-\psi ).\) 
